In lithographic printing, a so-called printing master such as a printing plate is mounted on a cylinder of the printing press. The master carries a lithographic image on its surface and a printed copy is obtained by applying ink to said image and then transferring the ink from the master onto a receiver material, which is typically paper. In conventional, so-called “wet” lithographic printing, ink as well as an aqueous fountain solution (also called dampening liquid) are supplied to the lithographic image which consists of oleophilic (or hydrophobic, i.e. ink-accepting, water-repelling) areas as well as hydrophilic (or oleophobic, i.e. water-accepting, ink-repelling) areas. In so-called “driographic” printing, the lithographic image consists of ink-accepting and ink-abhesive (ink-repelling) areas and during driographic printing, only ink is supplied to the master.
Printing masters are generally obtained by the so-called computer-to-film (CtF) method wherein various pre-press steps such as typeface selection, scanning, color separation, screening, trapping, layout and imposition are accomplished digitally and each color selection is transferred to graphic arts film using an image-setter. After processing, the film can be used as a mask for the exposure of an imaging material called plate precursor and after plate processing, a printing plate is obtained which can be used as a master. Since about 1995, the so-called ‘computer-to-plate’ (CtP) method has gained a lot of interest. This method, also called ‘direct-to-plate’, bypasses the creation of film because the digital document is transferred directly to a plate precursor by means of a so-called plate-setter.
Different technologies are being used in computer-to-plate. A is number of them are thermal technologies wherein thermal plates, sensitive to heat or infrared light, are widely used in computer-to-plate methods. Such thermal materials may be exposed directly to heat, e.g. by means of a thermal head, but preferably comprise a compound that converts absorbed light into heat and are therefore suitable for exposure by lasers, especially infrared laser diodes. The heat, which is generated on image-wise exposure, triggers a (physico-)chemical process, such as ablation, polymerization, insolubilization by cross-linking of a polymer, decomposition, or particle coagulation of a thermoplastic polymer latex, and after optional processing, a lithographic image is obtained. Many thermal plate materials are based on heat-induced coagulation of thermoplastic polymer particles.
EP-A 1 065 049 discloses a heat-sensitive material for making lithographic printing plates comprising on a lithographic support an image-forming layer comprising a hydrophilic binder, a cross-linking agent for a hydrophilic binder and dispersed hydrophobic thermoplatic polymer particles, and a covering layer comprising an organic compound comprising cationic groups.
EP-A 770 494 discloses a method wherein an imaging material comprising an image-recording layer of a hydrophilic binder, a compound capable of converting light to heat and hydrophobic thermoplastic polymer particles, is image-wise exposed, thereby inducing coalescence of the polymer particles and converting the exposed areas into an hydrophobic phase which defines the printing areas of the printing master. The press run can be started immediately after exposure without any additional treatment because the layer is developed by interaction with the fountain and ink that are supplied to the cylinder during the press run. During the first runs of the press, the non-exposed areas are removed from the support and thereby define the non-printing areas of the plate. This on-press processing method provides only a rapid clean-out (i.e. complete removal of the non-image areas of the coating) if first fountain is supplied to the plate and then also ink. However, it is difficult for the end-user to avoid that the plate surface gets in touch with ink (or with inked parts) before the plate is wetted by the dampening liquid on the press. Plate handlings which are critical for such plate contamination are e.g. plate loading, mounting the plate on the press, etc. It is difficult to develop those parts contaminated by ink in the on-press processing step, i.e. clean-out of the non-image parts is only achieved after a large number of revolutions of the plate cylinder. As a result, the latitude of plate handlings is limited to an unpracticle or even unacceptable level for the end-user.
U.S. Pat. No. 6,387,595 discloses a lithographic printing plate wherein a photosensitive layer capable of hardening or solubilization upon exposure to actinic radiation and an overcoat with a covarage of from 0.001 to 0.150 g/m2 which is soluble or dispersible in ink and/or fountain solution. The image-wise exposed material is on-press developable and the incorporation of this ultrathin overcoat provides excellent white light stability, high contrast, excellent ink receptivity and fast on-press development. However, the presence of such a thin overcoat of a water-soluble polymer, applied on the precursor of the present invention before image-wise exposing, has the drawback of reducing the sensitivity on heat-mode recording.
EP-A 816 070 discloses a heat sensitive imaging element comprising on a hydrophilic surface of a lithographic base an image forming layer including at least hydrophobic thermoplastic polymer particles and a compound capable of converting light into heat, and wherein, on the image forming layer, a covering layer is present having a thickness between 0.1 and 3 μm. However, there is no disclosure about a method wherein the covering layer is applied on the image forming layer after the exposure step and before processing the material.
EP-A 1 342 568 discloses a method of making a lithographic printing plate wherein an imaging material comprising an image-recording layer of a hydrophilic binder, a compound capable of converting light to heat and hydrophobic thermoplastic polymer particles, is image-wise exposed, thereby inducing coalescence of the polymer particles. The image-wise exposed material is processed by applying a gum solution to the image-recording layer, thereby removing non-exposed areas of the coating from the support. However, there is no disclosure about on-press processing with ink and fountain solution.